Ackermann geometry on race Spridgets


Ackermann geometry was developed in 1800(ish) to help carts turn corners. It is the geometry used on a standard Spridget but it can be improved for performance use.


When you go round a corner the outer wheel has to go further than the inner wheel so the inner wheel needs a bit more turn in. Ackermann geometry does that for you so when you turn left the wheels turn the correct amount.

It gives good tyre wear, predictable behaviour and good slow / tight cornering performance. It's a good choice for a do-everything road car which is why it has remained basically unchanged for over 200 years.

See more on Wikipedia


Have a look at Kimi's Ferrari above - the outer wheel turns in more than the inner wheel. Clearly this is not Ackermann geometry - what's going on?

Well on a race car you typically corner at high speed which transfers a lot of load to the outer tyre and leaves the inner tyre with much less load. In fact on a Spridget with too much anti-roll bar you will often see the inside front wheel completely off the ground! If the inside tyre does manage to stay in contact with the tarmac then you have more grip but with Ackermann you will have more angle too.

This matters because modern race tyres don't like low load and high slip angle, it makes the tyre scrub and causes tearing and overheating. In fact the optimum grip increases with slip angle and load up to its max point after which it will drop away. Most modern circuit racing tyres peak around 6 or 7 degrees. 

So if the outer tyre has a slip angle of 6 degrees on max load and the inner tyre has MORE angle from Ackermann but less load it won't do anything to add grip, it will just damage the tyre. 

The solution is to use Anti-Ackermann geometry which turns the outer wheel more than the inner. At least on a Formula 1 car...

On a Spridget?

Most Spridgets don't generate downforce and aren't running on super sensitive Pirelli slicks

But for track-focused Spridgets the standard Ackermann geometry does give away grip potential on semi-slicks and slicks. 

It also creates a turning centre at the midpoint of the rear axle which makes the car less stable. With the short wheelbase of a Spridget the turning-centre can be improved by moving backwards to increase high-speed stability. 

The answer for a Spridget  to improve the handling and maximise the grip potential of the tyres is reduction of the Ackermann angle. Mamba Motorsport have developed an Ackermann reduction kit, have a look in the shop.